Communications system, apparatus for creating a sub-channel and method therefor

ABSTRACT

A communications apparatus includes an input for receiving a data stream being transmitted from a first network node to a second network node using a main channel. A processing resource of the communications apparatus identifies data signifying an idle period within the data stream and determines whether the idle period is at least a suitable minimum duration to support initiating transmission of sub-channel data in place of at least part of the data signifying the idle period. Further, the processing resource is arranged to identify when the idle period is not of the suitable minimum duration and a need arises to transmit the sub-channel data within a predetermined period of time. In such circumstances, the processing resource sends a flow control message upstream to the first network node to halt transmissions therefrom, thereby generating the idle period of at least the suitable minimum duration.

The present invention relates to an apparatus for creating a sub-channelin a main channel of the type, for example, that supports communicationsbetween a first network node and a second network node. The presentinvention also relates to a communications system for supporting asub-channel within a main channel of the type, for example, thatsupports communications between a first network node and a secondnetwork node. The present invention further relates to a method ofcreating a sub-channel within a main channel of the type, for example,that supports communications between a first network node and a secondnetwork node.

BACKGROUND ART

In the field of network communications, it is known to implement passivemeasurement techniques at selected points in a communications network inorder to monitor Quality of Service levels and diagnose faults that canoccur from time-to-time in the communications network.

In this respect, it is known to deploy so-called “passive probes” at theselected points in the communications network. Such passive probes makemeasurements relating to network traffic travelling along one or morelinks in the communications network. Additionally, once collected,measurement data has to be communicated to, for example, a centralmonitoring station in the communications network for analysis andinterpretation.

In order to convey the measurement data from a passive probe to thecentral monitoring station, U.S. Pat. No. 2005/0083957 A1 proposes a lowbandwidth channel formed by inserting packets into a high bandwidthpacket stream. The packets are inserted at a predetermined interval,insertion causing latency that is recovered by minimising inter-packetgaps in the incoming high bandwidth channel. While the packets to beinserted are being transmitted, arriving high bandwidth packets arestored in an elastic buffer.

However, whilst the above technique provides a mechanism for achievingthe low bandwidth channel in the high bandwidth packet stream, it isdesirable to improve performance of the low bandwidth channel. Inparticular, the above-described technique relies upon the existence ofsufficiently large “gaps” in the high bandwidth data stream that can bereduced to allocate bandwidth to accommodate transmission time for thelow bandwidth channel. However, the above apparatus preserves trafficflow in the high bandwidth data stream as the traffic in the highbandwidth data stream is considered to be of greater importance than thetraffic using the low bandwidth channel. This is an overriding principleto which operation of the above apparatus adheres. Hence, ifinsufficient gaps exist in the high bandwidth data stream, transmissionson the low bandwidth channel have to be halted due to lack of bandwidthuntil a sufficiently large gap occurs in the high bandwidth stream.

As a result of a temporary incapability to transmit on the low bandwidthchannel, it is necessary to buffer the measurement data to betransmitted until the measurement data can be transmitted. Whilst theapparatus described above comprises the elastic buffer, there is a limitto elasticity of the buffer. To avoid running out of buffer capacity,the apparatus reaches a point where creation of new packets has to betemporarily halted until bandwidth becomes available to resumetransmission of the measurement data. Alternatively, the apparatusdiscards packets already created (and in the buffer) in order to providecapacity in the buffer for newly created packets containing measurementdata.

SUMMARY OF THE DISCLOSED EMBODIMENTS

According to a first aspect of the present invention, there is providedan apparatus for creating a sub-channel within a main channel between afirst network node and a second network node, the apparatus comprising:a data store for temporarily storing sub-channel data; a processingresource for monitoring downstream communication from the first networknode, the processing resource being arranged, when in use, to identify asuitable channel condition in the downstream communication, and todetermine whether insufficient time exists to await probabilisticoccurrence of the suitable channel condition before a deadline totransmit the sub-channel data is reached, the suitable channel conditionsupporting initiation of transmission of the sub-channel data in placeof at least part of data for signifying an idle period in the downstreamcommunications; wherein the processing resource is further arranged totransmit upstream, when in use, a flow control message for receipt bythe first network node, thereby causing timely generation of thesuitable channel condition before the deadline.

The processing resource may be arranged to identify, when in use, the atleast part of the data for signifying the idle period in the downstreamcommunication, and to determine whether insufficient time exists toawait probabilistic occurrence of a subsequent idle period before thedeadline to transmit the sub-channel data is reached, the subsequentidle period being required when the data for signifying the idle perioddoes not correspond to at least a predetermined suitable minimumduration to support the initiation of transmission of the sub-channeldata in place of the at least part of the data for signifying the idleperiod; and the timely generation of the suitable channel condition istimely generation of at least part of the subsequent idle period beforethe deadline, the at least part of the subsequent idle period having atleast the predetermined suitable minimum duration.

The processing resource may be further arranged to initiate transmissionof the sub-channel data in place of subsequent data for signifying theat least part of the subsequent idle period.

The processing resource may be arranged to determine whether the datafor signifying the idle period does not correspond to at least anaverage duration to support the initiation of transmission of thesub-channel data.

The apparatus may further comprise: memory operable to hold an extrapacket constituting the sub-channel data; memory operable to hold aparallel datastream while the extra packet is being sent; and controllogic.

The apparatus may further comprise a deserialiser operable to convert aserial datastream to the parallel datastream for the downstreamcommunications on the main channel, and a serialiser operable to convertthe parallel datastream back to the serial datastream.

The processing resource may be arranged to determine whether flowcontrol is supported for the downstream communications on the mainchannel. A time limit may be associated with the flow control message.

The processing resource may be arranged to allow the time limit toexpire.

The processing resource may be arranged to send a subsequent flowcontrol message for receipt by the first network node, therebyterminating the subsequent idle period. A substantially zero time limitmay be associated with the subsequent flow control message.

According to a second aspect of the present invention, there is provideda probe for monitoring signals in a communications network, the probecomprising the apparatus as set forth above in relation to the firstaspect of the invention.

According to a third aspect of the present invention, there is providedan interface converter module comprising the apparatus as set forthabove in relation to the first aspect of the invention.

According to a fourth aspect of the present invention, there is provideda communications system for supporting a sub-channel within a mainchannel, the system comprising: a first network node and a secondnetwork node for supporting the main channel therebetween; a data storefor temporarily storing sub-channel data; a network monitoring apparatusfor monitoring downstream communications from the first network node,the network monitoring apparatus being arranged to identify, when inuse, a suitable channel condition in the downstream communications, andto determine whether insufficient time exists to await probabilisticoccurrence of the suitable channel condition before a deadline totransmit the sub-channel data is reached, the suitable channel conditionsupporting initiation of transmission of the sub-channel data in placeof at least part of data for signifying an idle period in the downstreamcommunications; wherein the network monitoring apparatus is furtherarranged to transmit, when in use, a flow control message to the firstnetwork node, thereby causing timely generation of the suitable channelcondition before the deadline.

According to a fifth aspect of the present invention, there is provideda method of creating a sub-channel within a main channel between a firstnetwork node and a second network node, the method comprising:temporarily storing sub-channel data; monitoring downstreamcommunications from the first network node; identifying a suitablechannel condition in the downstream communications; determining whetherinsufficient time exists to await probabilistic occurrence of thesuitable channel condition before a deadline to transmit the sub-channeldata is reached, the suitable channel condition supporting initiation oftransmission of the sub-channel data in place of at least part of datafor signifying an idle period; and transmitting upstream a flow controlmessage for receipt by the first network node, thereby causing timelygeneration of the suitable channel condition before the deadline.

According to a sixth aspect of the present invention, there is provideda computer program element comprising computer program code means tomake a computer execute the method as set forth above in relation to thefifth aspect of the invention.

The computer program code element may be embodied on a computer readablemedium.

It is thus possible to provide a system, apparatus and method thereforthat are capable of creating opportunities to send the sub-channel datawhen at least one channel condition is insufficient, for example theidle periods detected are of insufficient length, to support initiationof transmission of the sub-channel data and a sufficiently long idleperiod cannot be awaited, for example due to a constraint, such as abuffer size. Data loss in relation to the sub-channel can therefore beobviated or at least mitigated. It is also possible to send, withminimal delay, high priority data, such as an alarm message, withouthaving to wait for sufficiently large gaps in the datastream of the mainchannel.

BRIEF DESCRIPTION OF DRAWINGS

At least one embodiment of the invention will now be described, by wayof example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a communications system comprising acommunications apparatus constituting an embodiment of the invention;

FIG. 2 is a schematic diagram of the communications apparatus of FIG. 1in greater detail;

FIG. 3 is a flow diagram of a method employed by the communicationsapparatus of FIG. 1 and FIG. 2; and

FIG. 4 is an event timing diagram of repeated invocations of the methodof FIG. 3.

DETAILED DESCRIPTION

Throughout the following description identical reference numerals willbe used to identify like parts.

Referring to FIG. 1, a communications system 100 comprises a firstnetwork node, for example a first host 102 capable of communicating witha second network node, for example a second host 104. The first host 102is therefore coupled to the second host 104 by a first unidirectionalcommunications link 106 in a first direction and a second unidirectionalcommunications link 108 in a second direction opposite to the firstdirection, thereby providing communications media for bi-directionalcommunications. In this example, the first host 102 is a first routerand the second host 104 is a second router, the first and second routerstogether providing connectivity between domains (not shown) in acommunications network (also not shown). However, the skilled personwill appreciate that the first and second hosts 102, 104 can be otherfunctional pairs of communications elements, for example an Ethernetcard in a Personal Computer and a router.

The first and second unidirectional communications links 106, 108 areeach supported, in this example, by a respective optical fibre. A firstmain communications channel is supported by the first unidirectionalcommunications link 106 and a second main communications channel issupported by the second unidirectional communications link 108.

In order to support a first sub-channel in the first main channel, and asecond sub-channel in the second main channel, an in-line sub-channelapparatus 110 of the type described in EP-A1-1 524 807 is disposed inthe first and second communications links 106, 108 between the first andsecond hosts 102, 104. Although the structure and operation of thein-line sub-channel apparatus 110 is well-documented in EP-A1-1 524 807,for the sake of ease of reference and ready understanding of theadditional and/or alternative functionality described later herein, thestructure of the in-line sub-channel apparatus 110 will now be brieflydescribed.

The in-line sub-channel apparatus 110 comprises a first sub-channelinjector 112 coupled to an application logic 114 that uses the firstsub-channel supported by the first sub-channel injector 112. In contrastwith EP-A1-1 524 807, the in-line sub-channel apparatus 110 alsocomprises a second sub-channel injector 116 coupled to the applicationlogic 114 as the application logic 114 also uses, in this example, thesecond sub-channel supported by the second sub-channel injector 116.Since the second sub-channel injector 116 is a reverse-directionimplementation of the first sub-channel injector 112, the secondsub-channel injector 116 will not be described further except in passingreference, since the skilled person will appreciate the structure andfunctions of the second sub-channel injector 116 from a description ofthe first sub-channel injector 112. Consequently, for the sake ofsimplicity and conciseness of description, the “first main channel” willnow be referred to as the “main channel”, and the “first sub-channel”will now be referred to as the “sub-channel” as no further referenceswill be made herein to the second main channel or the secondsub-channel.

Turning to FIG. 2, the application logic 114 is coupled to a datastreaminput 200 of the first sub-channel injector 112. The datastream input200 is also coupled to an input of an idle deletion module 202 and afirst input of a first multiplexer 204. An output of the idle deletionmodule 202 is coupled to a first input of a second multiplexer 206, asecond input of the second multiplexer 206 being coupled to an internalbuffer 207 of the application logic 114. An output of the secondmultiplexer 206 is coupled to an input of a First-In-First-Out (FIFO)buffer 208, an output of the FIFO buffer 208 being coupled to a secondinput of the first multiplexer 204.

Although not shown and not required if data is to be processed in serialform, a de-serialiser module is coupled before the datastream input 200to perform a serial-to-parallel conversion on incoming data arriving atthe first sub-channel injector 112 and a serialiser module is coupled toan output of the first multiplexer 204 to perform a parallel-to-serialconversion on outgoing data leaving the first sub-channel injector 112.

In operation, the communications system 100 supports a Gigabit Ethernetprotocol in accordance with the Institute of Electrical and ElectronicEngineers (IEEE) Standard 802.3 and the in-line sub-channel apparatus110 is capable of functioning in a manner described in EP-A1-1 524 807.However, the skilled person will recognize that the functionality of thein-line sub-channel apparatus 110 can be modified to include only someof the functionality described in EP-A1-1 524 807. Likewise, in thepresent example, the functionality of the in-line sub-channel apparatus110 can be modified to enhance functionality of the in-line sub-channelapparatus 110.

In this respect, the in-line sub-channel apparatus 110 is part of acommunications monitoring apparatus (not shown), for example a probe,such as any suitable probe for measuring network performance, thatgenerates measurement data in relation to a given communications link,for example the first unidirectional communications link 106. In thisexample, the measurement data generated has to be forwarded to a centralmonitoring station for analysis in order to monitor Quality of Serviceof, inter alia, the communications system 100 and diagnose any faults.The measurement data generated has to be stored temporarily aspacketised data by the application logic 114 for onward transmission.However, the storage capacity of application logic 114 is finite and theapplication logic 114 has to await suitable channel conditions in orderto be able to inject at least one packet into a datastream on the mainchannel, the datastream being transmitted from the first host 102 to thesecond host 104.

When the first host 102 does not need to communicate with the secondhost 104, the first host 102, instead of simply remaining inactiveduring an idle period, sends data signifying the idle period in thedatastream to the second host 104 in accordance with the IEEE 802.3standard. As described in EP-A1-1 524 807, the in-line sub-channelapparatus 110 exploits idle periods on the main channel to support thesub-channel.

Turning to FIG. 3, the application logic 114 monitors the internalbuffer 207 in order to determine (300) whether the internal buffer 207has packets containing measurement data to be sent. If the internalbuffer 207 is empty, the application logic 114 continues monitoring thestatus of the internal buffer 207. Referring to FIG. 4, in accordancewith a by-pass mode, an incoming frame of data 400 is received (402) bythe in-line sub-channel apparatus 110. The incoming frame of data 400 ispassed to the first multiplexer 204 without further interference, andhence delay, caused by the application logic 114, whereupon an outputframe 403, constituting an unmodified version of the incoming frame ofdata 400, is sent by the in-line sub-channel apparatus 110 for receipt(404) by the second host 104. Referring back to FIG. 3, in the eventthat the internal buffer 207 contains one or more packet to betransmitted using the sub-channel, the application logic 114 determines(302) whether conditions on the main channel are suitable to supportinitiation of transmission of at least one packet being stored by theinternal buffer 207. Consequently, the data signifying the idle periodmust be detected and, for example, occurrences of idle periods on themain channel may not be of sufficient length to support initiation oftransmission of at least part of the sub-channel data, i.e. data storedby the internal buffer 207. Another condition that may need to be met(depending upon system requirements) is whether a so-called “holdtimer”, as described in EP-A1-1 524 807, has expired. Additionally oralternatively, the condition can be whether a detected idle periodexceeds a calculated average duration. In this example, for the sake ofsimplicity of description, the application logic 114 only verifies if adetected idle period, identified by data codes conforming to the IEEE802.3 standard, is greater than a predetermined minimum suitableduration.

If the idle period is greater than the predetermined minimum suitableduration, the in-line sub-channel apparatus 110 sends (306) the at leastpart of the sub-channel data in accordance with the technique describedin EP-A1-1 524 807 and depending upon the capacity of the FIFO buffer208. On the other hand, if the conditions on the main channel are notsuitable, for example the above minimum duration condition has not beenmet, the application logic 114 determines (306) whether waiting apredetermined delay period is permissible, for example without resultingin overflow of the internal buffer 207. The occurrence of the suitablechannel condition is, of course, probabilistic. However, if it ispossible to wait the predetermined delay period, the application logic114 abstains (307) from sending the at least part of the sub-channeldata for the predetermined delay period and then reverts to determining(302) whether the conditions on the main channel are now suitable forimplementing the sub-channel.

If it is not possible to delay transmission on the sub-channel without aresulting overflow of the internal buffer 207, the application logic 114determines (308) whether a flow control message has been sent to thefirst host 102, for example a Media Access Control (MAC) Pause frame.The above use of flow control in the communications system 100 isaspirational on the part of the in-line sub-channel apparatus 110 and soverification that the flow control message has been sent has to takeplace in order to ascertain whether the flow control is being employedbetween the first and second hosts 102, 104. In this respect, symmetricflow control can be in operation if flow control is supported by boththe first and second hosts 102, 104 or asymmetric flow control can be inoperation if flow control is only supported by the first host 102.However, if only the second host 104 supports flow control or neitherthe first host 102 nor the second host 104 support flow control, thenthe in-line sub-channel apparatus 1 10 has to provide a mechanismalternate to that described herein to avoid overflow of the internalbuffer 207, otherwise overflow is risked. The application logic 114 can,of course in another embodiment, be configured to detect whether thefirst host 102, the second host 104, both or neither support flowcontrol by monitoring, for example an autonegotiation procedure betweenthe first and second hosts 102, 104 at start-up of the first and/orsecond host 102, 104, but such a monitoring facility is not essentialand the implementation described herein based upon an optimisticattitude to implementation of flow control is adequate.

In the event that control of the datastream transmitted by the firsthost 102 is not possible, the application logic 114 is implicitly“aware” of the lack of flow control support by the first host 102 byvirtue of the flow control message already having been sent. Recordal ofprior use of the flow control message is recorded by a flag (not shown).

Hence, if the flow control message has already been sent once and theconditions on the main channel remain unchanged, flow control is assumedcurrently not to be implemented by the first host 102 and theapplication logic 114 has no choice but to drop (310) a packet from theinternal buffer 207 in order to avoid overflow thereof (assuming noalternative mechanism has been implemented). The flag is then cleared(312) by the application logic 114 and the application logic 114 revertsto determining (300) whether further packets need to be sent, but arecurrently stored in the internal buffer 207.

Alternatively, if the flag has not been previously set, indicting thattransmission of the flow control message has not been attempted inrespect of the at least part of the sub-channel data that needs to besent, the application logic 114, using the second sub-channel injector116, sends (314) the flow control message 406 (FIG. 4) containing apause duration of a value, for example, sufficiently large to enabletransmission of the at least part of the sub-channel data to the firsthost 102, and then sets (316) the flag to indicate that the flow controlmessage has been sent and then awaits at least one suitable channelcondition, for example detection of idle code groups.

In response to the MAC Pause frame, the first host 102 finishes sendingany frames currently in the process of transmission and then sendsso-called IDLE characters in place of frames of data, i.e. datasignifying a subsequent idle period, corresponding to the pauseduration.

The application logic 114, which has now returned to determining (302)whether conditions on the main channel are now suitable for sending theat least part of the sub-channel data, detects the IDLE charactersreceived, the IDLE characters corresponding to the pause duration thatis greater than the predetermined minimum suitable duration.Consequently, the in-line sub-channel apparatus 110 is able to send(306) the at least part of the sub-channel data in a manner described inEP-A1-1 524 807. In this example, a first packet 408 containing firstmeasurement data is sent, followed by a second packet 410 containingsecond measurement data. Thus, timely removal of data from the internalbuffer 207 is achieved, thereby avoiding overflow of the internal buffer207.

Referring back to FIG. 4, once sufficient sub-channel data to provideoverflow relief to the internal buffer 207 has been sent, the in-linesub-channel apparatus 110 can revert to the operation described inEP-A1-1 524 807 (modified in whatever way so desired) as sufficient timenow exists to await suitable conditions on the main channel. In thisrespect, the in-line sub-channel apparatus 110 can continue implementingthe pass-through mode to let data frames 412 transmitted by the firsthost 102 to the second host 104 pass through the in-line sub-channelapparatus 110 without delay.

Any subsequent need 414 to force idle periods in the datastream from thefirst host 102 can be implemented in the manner already described above.

However, excessively long pause durations or excessive use of flowcontrol messages within a given period of time can result in excessivedropping of packets or permanent changes to routing tables by routers asa result of an inference by the routers of an existence of a downstreamcommunications problem. Therefore, the application logic 114 can bearranged to estimate a required pause duration for injecting one or morepackets that needs to be sent. If appropriate, the estimated pauseduration is incorporated into the Pause control frame. The estimatedpause duration can be deemed appropriate if a suitable predeterminedinterval of time has elapsed since transmission of a previous Pausecontrol frame.

Of course, it should also be appreciated that if the in-line sub-channelapparatus 110 finishes transmitting sub-channel data before expiry ofthe pause duration, a subsequent flow control message can be sent to thefirst host 102 having a reduced pause duration of substantially zerotime, the reduced pause duration superseding the existing pause durationbeing implemented and resulting in resumption of transmission of dataframes from the first host 102 to the second host 104 in accordance withthe IEEE 802.3 standard.

Although the above examples have been described in the context of packetcommunications, it should be appreciated that the term “message” isintended to the construed as embracing packets, datagrams, frames, cellsand/or protocol data units and so these terms should be understood to beinterchangeable. Therefore, in the context as described herein, itshould be understood that a “packet” is not just an IP packet but it isa frame that can contain an IP packet. In the case of Ethernet, it is anEthernet frame that may already be encoded, for example using 8B/10Bencoding. The type of encoding will depend on encoding techniques usedby passing traffic on the main channel.

Whilst it has been suggested above that the in-line sub-channelapparatus is implemented in a probe, the skilled person will appreciatethat the in-line sub-channel apparatus 110 can be implemented in variousforms, for example in a highly integrated form suitable for replacingindustry-standard interface converter modules, for example those knownas GigaBit Interface Converters (GBICs). Current GBICs are effectivelytransceivers that translate signals of one media type, for exampleoptical or electrical, to another media type. By providing a replacementGBIC including the in-line sub-channel apparatus, numerous applicationsrequiring sub-channels are further enabled.

Alternative embodiments of the invention can be implemented as acomputer program product for use with a computer system, the computerprogram product being, for example, a series of computer instructionsstored on a tangible data recording medium, such as a diskette, CD-ROM,ROM, or fixed disk, or embodied in a computer data signal, the signalbeing transmitted over a tangible medium or a wireless medium, forexample, microwave or infrared. The series of computer instructions canconstitute all or part of the functionality described above, and canalso be stored in any memory device, volatile or non-volatile, such assemiconductor, magnetic, optical or other memory device.

1. An apparatus for creating a sub-channel within a main channel betweena first network node and a second network node, the apparatuscomprising: a data store for temporarily storing sub-channel data; aprocessing resource for monitoring downstream communication from thefirst network node, the processing resource being arranged, when in use,to identify a suitable channel condition in the downstreamcommunication, and to determine whether insufficient time exists toawait probabilistic occurrence of the suitable channel condition beforea deadline to transmit the sub-channel data is reached, the suitablechannel condition supporting initiation of transmission of thesub-channel data in place of at least part of data for signifying anidle period in the downstream communications; wherein the processingresource is further arranged to transmit upstream, when in use, a flowcontrol message for receipt by the first network node, thereby causingtimely generation of the suitable channel condition before the deadline.2. An apparatus as claimed in claim 1, wherein the processing resourceis arranged to identify, when in use, the at least part of the data forsignifying the idle period in the downstream communication, and todetermine whether insufficient time exists to await probabilisticoccurrence of a subsequent idle period before the deadline to transmitthe sub-channel data is reached, the subsequent idle period beingrequired when the data for signifying the idle period does notcorrespond to at least a predetermined suitable minimum duration tosupport the initiation of transmission of the sub-channel data in placeof the at least part of the data for signifying the idle period; and thetimely generation of the suitable channel condition is timely generationof at least part of the subsequent idle period before the deadline, theat least part of the subsequent idle period having at least thepredetermined suitable minimum duration.
 3. An apparatus as claimed inclaim 2, wherein the processing resource is further arranged to initiatetransmission of the sub-channel data in place of subsequent data forsignifying the at least part of the subsequent idle period.
 4. Anapparatus as claimed in claim 2, wherein the processing resource isarranged to determine whether the data for signifying the idle perioddoes not correspond to at least an average duration to support theinitiation of transmission of the sub-channel data.
 5. An apparatus asclaimed in claim 1, further comprising: memory operable to hold an extrapacket constituting the sub-channel data; memory operable to hold aparallel datastream while the extra packet is being sent; and controllogic.
 6. An apparatus as claimed in claim 1, wherein the processingresource is arranged to determine whether flow control is supported forthe downstream communications on the main channel.
 7. An apparatus asclaimed in claim 1, wherein a time limit is associated with the flowcontrol message.
 8. An apparatus as claimed in claim 7, wherein theprocessing resource is arranged to allow the time limit to expire.
 9. Anapparatus as claimed in claim 2, wherein the processing resource isarranged to send a subsequent flow control message for receipt by thefirst network node, thereby terminating the subsequent idle period. 10.An apparatus as claimed in claim 9, wherein a substantially zero timelimit is associated with the subsequent flow control message.
 11. Aprobe for monitoring signals in a communications network, the probecomprising the apparatus as claimed in claim
 1. 12. An interfaceconverter module comprising the apparatus as claimed in claim
 1. 13. Acommunications system for supporting a sub-channel within a mainchannel, the system comprising: a first network node and a secondnetwork node for supporting the main channel therebetween; a data storefor temporarily storing sub-channel data; a network monitoring apparatusfor monitoring downstream communications from the first network node,the network monitoring apparatus being arranged to identify, when inuse, a suitable channel condition in the downstream communications, andto determine whether insufficient time exists to await probabilisticoccurrence of the suitable channel condition before a deadline totransmit the sub-channel data is reached, the suitable channel conditionsupporting initiation of transmission of the sub-channel data in placeof at least part of data for signifying an idle period in the downstreamcommunications; wherein the network monitoring apparatus is furtherarranged to transmit, when in use, a flow control message to the firstnetwork node, thereby causing timely generation of the suitable channelcondition before the deadline.
 14. A method of creating a sub-channelwithin a main channel between a first network node and a second networknode, the method comprising: temporarily storing sub-channel data;monitoring downstream communications from the first network node;identifying a suitable channel condition in the downstreamcommunications; determining whether insufficient time exists to awaitprobabilistic occurrence of the suitable channel condition before adeadline to transmit the sub-channel data is reached, the suitablechannel condition supporting initiation of transmission of thesub-channel data in place of at least part of data for signifying anidle period; and transmitting upstream a flow control message forreceipt by the first network node, thereby causing timely generation ofthe suitable channel condition before the deadline.
 15. A computerprogram element comprising computer program code means to make acomputer execute the method of claim
 14. 16. A computer program codeelement as claimed in claim 15, embodied on a computer readable medium.